This application claims priority to Great Britain Patent Application No. 0007033.4, filed Mar. 23, 2000.
The invention relates to microwave heating apparatuses, and methods of heating articles using such apparatuses.
When an object is defrosted in a conventional microwave oven, the initial microwave heating effect causes thawing of ice a small distance into the item to be defrosted, producing regions of free water molecules. Because the absorption of microwave energy is much higher in water than in ice, this causes localised heating. In extreme cases it is possible to fully cook the product where the ice has initially melted, while leaving the remaining ice frozen. In the case of a food product which must be stored frozen, and served hot to a customer, for example a burger, this can lead to the situation where the customer is presented with a food product which is apparently correctly cooked and heated, but where certain areas of the product have not attained the legally required temperature before serving. FIG. 1 demonstrates such a situation.
The conventional methods of attempting to overcome this problem come in two forms: introducing a time delay into the thawing process, or shaping the product to maximise the surface area and thus the absorption of microwave energy.
The main benefit quoted for microwave heating is the increase in speed over conventional heating methods. If the time delay method is used to overcome the problem mentioned above, time is allowed during the heating process for thermal conduction to transfer some of the heat from the thawed regions to those which are still frozen; i.e. thawing by conduction as in any conventional method. The delay which is introduced into the heating process is usually performed by operating the magnetrons supplying the microwaves at a reduced duty cycle, i.e. pulsing the magnetrons on and off. A typical ratio of xe2x80x9conxe2x80x9d to xe2x80x9coffxe2x80x9d time is eight seconds xe2x80x9conxe2x80x9d followed by twelve seconds xe2x80x9coffxe2x80x9d, which gives an effective reduction to only 40% of the available microwave power, and thus increases the time required to defrost the product by a factor of approximately 2xc2xd times. Particularly in commercial xe2x80x9cfast foodxe2x80x9d applications, this time delay is unacceptable.
In a domestic situation, much use is made of ring shaped cooking containers, the large diameter hollow centre allowing the microwaves to penetrate the product from two sides. This toroidal shape does indeed minimise the problems of ice formation, but at the cost of ease of putting the product into the cooking container. This also has the effect that the product is bulky to store whilst frozen.
One process which has heretofore been considered largely unsuitable for microwave heating is that of xe2x80x9ctemperingxe2x80x9d foodstuffs, i.e. raising the temperature of the product from xe2x80x9cdeep frozenxe2x80x9d (usually considered to be xe2x88x9218xc2x0 C.) to a xe2x80x9csofterxe2x80x9d frozen temperature of about xe2x88x924xc2x0 C. A particular example of this is the tempering of blocks of meat products to allow mechanical operations, such as slicing to produce evenly thin slices of meats for use in ethnic food preparation. This process usually highlights all of the inadequacies of conventional microwave heating, as the localised melting mentioned above proves disastrous in such a case. Once thawed or tempered, the food product may also then require raising in temperature to a serving condition, possibly also with the addition of extra heating by a conventional means for cosmetic xe2x80x9cbrowningxe2x80x9d purposes, without further intervention from the operator of the microwave apparatus.
These problems are alleviated by ovens described in the applicant""s earlier British Patent Application No. 9915368.6, filed Jul. 2, 1999, which describes the use of separate internal and external antennae for irradiating food both internally and externally.
British Patent No. 1,470,408 describes a microwave oven in which food is heated internally by a rod which passes through the food, and externally by a plate member, both of which are connected to a single magnetron. However, this may result in the food cooking too quickly from the inside relative to the outside, or vice versa, particularly when different sizes, shapes and types of food are cooked in the oven.
It should be understood that xe2x80x9cfoodxe2x80x9d in the present specification includes any type of food or drink. Furthermore, xe2x80x9cantennaxe2x80x9d in this specification includes any article or part of an article from which radiation is emitted, and includes for example part of a magnetron from which microwave radiation is emitted.
According to the invention there is provided a microwave heating apparatus and assembly, and a method of heating food, as set out in the accompanying claims.